1. Field of the Invention
The present invention relates to a method of selecting a cell formed by a base station to which a mobile station moving in a radio service area covered by cells is to be connected, and a mobile communications terminal for selecting a cell to communicate by radio with a base station.
2. Description of the Background Art
In recent years, a mobile communications system has been developed, in which communications including telephone or speech transmission are accomplished by using mobile telephones, such as PHS (Personal Handyphone System) and cellular phones. For example, in a digital mobile communications system employing an access scheme, such as a TDMA (Time Division Multiple Access) or a CDMA (Code Division Multiple Access), communications are established between base stations located in the service area and radio terminals serving as mobile stations, to thereby enable communications including telephone to be established between mobile stations or between a mobile station and other kinds of communications terminal.
In such a communications system, the entire service area is divided into a plurality of small radio zones called cells. The cells are formed by respective base stations to provide service to mobile stations positioned in a service area. In such a case, the base stations are disposed so as to have the respective cells thereof partially overlapped with each other to thereby establish a continuous service area. A mobile station will, when moving or carried into another cell during communication with a current base station, carry out handover for switching the cells and be able to continue, after switching, communication with a new base station forming the other cell. It is therefore necessary to provide the mobile station with a cell selecting function of selecting a cell to be switched when the station travels between cells in order to maintain good communication quality.
With the cell selecting function, for example, a cell associated with the strongest radio wave received is determined as a proper cell among the radio waves received from the base stations to communicate with the base station forming that cell to carry out process control, such as location registration to continue telephoning and communications. The mobile stations are thus adapted to monitor the level or intensity of signals received from the base station currently connected and from other base stations not in connection therewith to determine a base station that has transmitted the received signal which has its level highest among them, and then switch to a cell formed by that base station as a cell to which the mobile station will be connected, thus completing handover. Publication, EP 1 069 794 A2, for example, discloses a cell search control method for selecting the best base station for a mobile station.
Thus, conventionally, the electric power of the received signals was monitored to select a cell associated with the power of a received signal higher than others. With such conventional systems, for example, when an obstacle or the like is temporarily involved on a propagation path between the base and mobile stations to significantly attenuate the radio waves on the path, handover has to be made to another base station. In this way, since cell switching is carried out upon the power of a received signal temporary fluctuated, it was a problem that, even when a mobile station comes to a place where the propagation distance is shorter to the base station so as to sufficiently secure, but temporarily reduce, the electromagnetic field of a received signal, handover may excessively often occur for switching to another base station having its propagation distance longer than the current one and the field of the received signal temporarily stronger at a moment.
When such unnecessary handover takes place excessively, processing loads increase too much in the entire communications system, base stations and mobile stations, thus causing increase in electric power consumption, as was a drawback.